The transmission of information over optical fibers is becoming pervasive. This is motivated, at least in part, because optical fiber offers much larger bandwidths than electrical cable. Moreover, optical fiber can connect nodes over large distances and transmit optical information between such nodes at the speed of light.
There are, however, a number of physical effects that limit the ability to transmit large amounts of information over an optical fiber. One such effect is called “chromatic dispersion,” which refers to the spreading of a pulse of light (i.e., an “optical signal” or “lightwave signal”) due to the variation in the propagation velocity of the different optical frequencies (or equivalently, wavelengths) making up the pulse.
Chromatic dispersion has two root causes. The first is due to the fact that silica of the optical fiber, like any optical material, has an index of refraction that is frequency-dependent. This is referred to as “material dispersion.” The second cause is due to the nature of the propagation of light down the fiber and is referred to as “waveguide dispersion.” The power distribution of the light between the core and the cladding of the fiber is a function of frequency. This means the “effective index” or “propagation constant” of the waveguide mode is a function of frequency as well, which causes the optical signal to disperse as it travels down the fiber.
In optical fiber communication systems, chromatic dispersion causes individual bits to broaden, since each bit is composed of a range of optical frequencies that separate due to their different propagation velocities. Such broadening eventually leads to intersymbol interference due to overlap of adjacent bits, which results in unacceptable data transmission errors. Chromatic dispersion compensation is usually needed to obtain the required performance in lightwave transmission systems operating at per channel data rates of 10 Gb/s or above. For example, the dispersion of a standard single mode fiber (SMF) at the key lightwave communications wavelength of 1550 nm is roughly 17 ps/nm-km. For a 10 Gb/s transmission system, the optical bandwidth per channel is typically a minimum of 0.1 nm, and is often greater. Transmission through a 30 km span of SMF would lead to a chromatic dispersive broadening of the signal of 51 ps, which is 50% of the bit period (100 ps).
Such a broadening is unacceptably large and would lead to a large error rate. The problem becomes much more acute with higher data rates, such as 40 Gb/s per channel systems currently under development. The problem will even become more acute for the anticipated higher data rate systems presently being contemplated. Further details about the nature of chromatic dispersion in optical fibers and the consequences for optical networks can be found in the book by Ramaswami and Sivarajan, entitled Optical Networks, a Practical Perspective, Morgan Kaufmann Publishers, in chapter 2.3.
Efforts have been made in the past to develop systems and methods for compensating for the effects of chromatic dispersion. For example, dispersion-compensating fibers (DCF) have been developed that have the opposite sign of dispersion compared to conventional single mode fibers have been developed and are widely deployed as compensators. However, the DCF technique lacks the ability to easily fine tune the spectral variation of the dispersion and involves a relatively large insertion loss for long fiber links. Chirped fiber Bragg gratings can also compensate fixed amounts of dispersion, but only for one WDM channel at a time. Both techniques lack the ability to reprogram or programmably fine tune the amount of dispersion and its spectral profile, which is likely to be needed to develop higher rate lightwave communication systems.
A number of workers have used programmable pulse shapers to programmably compensate chromatic dispersion in high-power femtosecond pulse amplifiers and in nonlinear optical pulse compression systems. A variety of spatial light modulator (SLM) types have been used, including liquid crystals, acousto-optic modulators, and deformable mirrors.
By way of examples, the use of a deformable-mirror SLM to correct chromatic dispersion is described in the paper by E. Zeek et al., Pulse compression by use of deformable mirrors, Opt. Lett, 24, 493-495 (1999). The use of an arrayed waveguide grating (AWG) rather than a bulk diffraction grating as the spectral disperser is described in the paper by H. Takenouchi et al., entitled 2×40-channel dispersion-slope compensator for 40-Gbit/s WDM transmission systems covering entire C- and L-bands, presented at the Optical Fiber Communications Conference (OFC), sponsored by the Optical Society of America, Anaheim, Calif., March 2001; however, in this paper a fixed phase mask is used in place of an SLM, with the result that the dispersion is not programmable. Further, the article by C. Chang et al. entitled Dispersion-free fiber transmission for femtosecond pulses by use of a dispersion-compensating fiber and a programmable pulse shaper, Opt. Lett. 23, 283-285 (1998) describes chromatic dispersion compensation using a liquid crystal SLM.
These and the other efforts described in the cited references all have the shortcoming that the operation of the dispersion compensation system depends on the SOP and/or that the system is not sufficiently programmable to handle the dispersion slope and higher-order dispersion terms or to reprogram the dispersion profile to accommodate changes in the length of optical fiber links in a switched optical networking environment. The dependence of a chromatic dispersion compensation system on the SOP of the input lightwave is major shortcoming because the SOP of light having traveled through an optical fiber system is scrambled and can vary with time, resulting in polarization-dependent loss (PDL). Further, the inability to robustly perform phase encoding of the signal reduces the ability to accurately compensate for the chromatic dispersion characteristics of a given optical fiber system.
Accordingly, what is needed is a system and method that can programmably compensate, with a high degree of accuracy, an optical signal for chromatic dispersion effects of an optical fiber, while also being insensitive to the SOP of the light signal being processed.